Grid bias and screen control for electronic amplifiers



March 16, 1965 J. LOVE 3,174,113

GRID BIAS AND SCREEN CONTROL i OR ELECTRONIC AMPLIFIERS Filed Feb. 17. 1961 OUT PUT INVENTOR.

United States Patent 3,17 4,113 GRID BIAS AND SCREEN CONTROL FOR ELECTRONIC AIVIPLIFIERS John Love, 37 Boehrn Ave., Mount Tabor, NJ. Filed Feb. 17, 1961, Ser. No. 89,997 7 Claims. (Cl. 330128) This invention relates to radio frequency and audio frequency amplifiers and particularly to a simplified grid bias voltage supply system and an improved method to control and protect the screens of screen grid electronic tubes.

Heretofore, negative grid bias, for triodes and multielement tubes in radio frequency amplifiers has been usually supplied by means of some form of resistor in the grid circuit, using the voltage drop across said resistor when grid current flows due to drive from the stage ahead. The disadvantage of this system is that any resistance in the grid circuit must be overcome by the driving means and much of the driving power is, therefore, dissipated across this said resistor. Furthermore, grid bias exists only when drive is applied to the grid of the amplifier tube causing grid current to flow. Therefore, no grid bias exists without grid current flowing. A further disadvantage of this system is that the bias is not constant as it changes substantially proportional to grid current flow.

Another form of grid bias is placing a resistor in the cathode circuit. The above also applies to this system plus the fact that this said cathode resistor is also in the plate circuit as well as in the associated grid circuit and, therefore, dissipates a substantial amount of plate power as well as grid drive power. Although the cathode type of grid bias resistor functions to a certain extent Without grid current flow, it, nevertheless, is continuously changzero as is the case of the combination hereinafter described. A further disadvantage is that it has a degenerative effect which reduces the overall efficiency of the amplifier.

The use of dry batteries as a bias means is satisfactory to a certain extent for a short period of time. Current flowing through them against their polarity deteriorates them rapidly. Batteries available today for bias purposes have a high internal resistance that increases with use and with age and this resistance dissipates a substantial amount of the grid driving power.

When a power pack is used, a transformer, a rectifier, a filter capacitor, a choke and a bleeder combination is required. The bleeder resistor which dissipates the total output of the said power pack also dissipates a substantial amount of grid driving power.

Grid number two, in multi-elernent tubes, hereinafter referred to as the screen grid, also influences the amount of plate current flow. For screen protection and plate current control, prior inventors have resorted to mechanical relays, voltage regulator tubes in series and clamp tube means.

The disadvantage of the mechanical relays is their inherent inertia and the differential between opposite functions which prevents modulation of the screen with the plate. When a series of voltage regulator tubes, for holding the screengrid to a desired potential, is used, the screen grid cannot be successfully modulated with the plate, as the screen modulation voltage extremities are discharged to ground across the said voltage regulator tubes. With the use of the present form of clamp tube or tubes, at high value of grid resistor must be used in the associated grid circuit so that, during grid drive, there is a sufiicient voltage drop across this said grid resistor that when applied to the clamp tube grid it cuts off the plate current of the said clamp tube. The disadvantage of this method is the increased drive necessary due to the drive loss across the said high value of grid bias resistor.

A primary object of this invention is to provide a simplified means to maintain a constant grid bias that would put a relatively small amount of resistance in the grid circuit in which it is used without the use of any form of grid bias resistor, cathode resistor, battery or bias power pack.

Another object of this invention is to provide a clamp tube grid control means that does not require the usual high value of grid bias resistor, a clamp tube that will function without removing bias from the associated grid circuit.

A further object of this invention is to provide, in conjunction with the above mentioned clamp tube control, a means to manually control the idle plate current of the amplifier tube or tubes in the associated circuit.

A still further object of this invention is to provide a means to prevent any tendency of self-oscillation in screen grid power amplifiers.

A still further object of this invention is to provide an improved bias method that is applicable to all kinds of equipment wherein electronic tubes are used.

Other objects and advantages of this invention will be obvious from the following description.

In the drawing, FIG. 1 illustrates, by way of 'blockand schematic circuit diagram, a radio frequency transmitter final output stage. For the sake of simplicity all filament and plate power supplies, plate and screen modulating means and driving means have been omitted.

Referring to the drawings, the block diagram 1 contains a complete grid bias system which comprises a condenser 2 and an inverse breakdown diode 3 of the Zener type. Two factors account for the successful operation of this bias combination. The first is the normal'disposition of the open grid of a triode to ccumulate a negative charge with respect to its cathode, sufiicient to substantially cut off its plate current. The second is the ability of an electrolytic capacitor to store power beyond the rating of its actual capacity. It might be stated that it acts as a chargeable storage battery to a certain extent. It will, therefore, be observed from the foregoing that placing a diode of the class described or a voltage regulator tube in the grid circuit of a triode will effectively isolate the said grid which will continue to build up a negative charge to the spill over or breakdown voltage point of the said diode or voltage regulator tube. Connecting an electrolytic capacitor 2 across the said diode 3 or voltage regulator tube assists this negative voltage build up as it is recharged by alternate pulses of the applied grid drive and assists in the bias maintenance in the absence of the said grid drive. In view of the fact that, at or beyond, the inverse voltage breakdown point of either a diode of the class described or a standard type of voltage regulator tube, the internal ohmic resistance drops to a matter of only a few ohms compared to the thousands of ohms associated with the systems heretofore mentioned. This is characteristic of both components.

A non-electrolytic capacitor may be used in place of the electrolytic capacitor 2 as shown and described. However, in practical application, the advantage of the electrolytic type as above described is obvious. The said diode or the said voltage regulator tube may be used in the cathode circuit with or without parallel capacity due to the added plate current flow through it.

With the use .of a screen grid amplifier tube 4, as is shown in the drawing, the grid 5 has not complete con trol of the plate current only in conjunction with the screen grid 6.

This invention discloses clamp tube grid control circuitry that requires only a few ohms in the grid circuit for actuation and control of the clamp tube 7. This said circuitry operates effectively without influencing the operation of the grid bias systems 1,2 and 3 heretofore described as will be easily observed from the drawing.

This clamp tube grid control circuit is primarily a two stage voltage amplifier using a pair of triodes 7 and 8 each acting as its own rectifier and using alternate half cycles of the alternating current plate supply from the secondary 10 of the transformer 11. The primary 12 which is also the low voltage filament supply for tubes 7, 8 and 9, is center tapped and connected across the secondary circuit 10 through and between resistors 13 and 14. The purpose of this arrangement is to ground the said transformer primary 12 in such a manner as to place a minimum of voltage potential between filament and cathode of the said tubes 7, 8 and 9. With the proper value of plate load resistors 15 and 16, the amplification factor of this two tube combination is so high that one volt negative applied to grid 17 of tube 8 will indirectly and completely cut off the plate current of the clamp tube 7. Therefore, only relative few ohms are required in the resistor. 18.

The operation of the two stage voltage amplifier tubes 8 and 9 and their influence on the clamp tube 7 is as follows. Any current flow in the grid circuit of the amplifier tube 4 must pass through the resistor 18 which is also in the grid circuit of the first amplifier tube 8, said grid current flow through the said resistor 18 puts a negative bias on grid 17 of the said tube 8 thereby cutting off its plate current, therefore no current will flow through its plate load resistor 16 which will then have no voltage difference across it thereby causing the grid and cathode of tube 9 to be at the same potential, which would be zero bias, thus unblocking the plate circuit of the said tube 9 causing current to flow through its plate load resistor 15 which puts a much amplified negative bias on the grid of the clamping tube 7 blocking its plate circuit which is the screen grid draining circuit. To sum this up in simple language, when the grid 17 of the first tube 8 is negative so, also, will be. grid of the clamping tube 7 but amplified many fold. When no grid current flows through resistor 18, said resistor would have no voltage difference across it thereby putting a zero bias on grid 17 of tube 8 and substantially simultaneously and indirectly. put a zero bias on the clamping tube 7 which would unblock the said clamping tube 7 and drain the screen grid 6 to substantially zero voltage when the variable resistor 19 is set at Zero resistance. The capacitors across resistors 15 and 16 act as filter condensers to store enough energy to carry over alternate half cycles but they must be no lagrer than -.01 microfarad or there would be a time lag in the end results. This two stage voltage amplifier, as described, will be known in my claims as a direct current voltage amplifying means and the term cathode, in my claims, will mean any form or arrangement of filament etc. that would function the same as a cathode. Also, the phrase voltage regulating device means a Zener diode, a voltage regulating tube or anything that could perform such a function.

It is therefore obvious from the above that only a minimum of grid current flow across resistor 18 is required i to cut off the plate current of the clamp tube 7. Manual Cit control of the amount of idle plate current of theamplifier tube 4 is accomplished by the use of the variable resistor 19 which varies the bias on the grid 20 of the said clamp tube 7 due to the voltage drop across the used part of the resistance of the said variable resistor 19. Itmust be herewith understood that, although the above described clamp tube grid control voltage amplifier is used in this invention, any form of amplification, including transistors, could be used and still come within the scope of this invention. Although the foregoing has specifically referred to radio frequency amplifiers, all the functions heretofore described are similarly applicable to audio frequency amplifiers.

Heretofore, class B push pull audio screen grid power amplifiers usually required a source of power for the screen grids other than the plate power supply. With the use of the system heretofore described, the screen current can be taken off the plate power supply through a voltage dropping resist-or determined by the particular tube specifications wtihout the necessity of a high value of resistance in the grid circuit.

When a pair of screen grid tubes is used in a push pull audio amplifier and suflicient negative bias is applied to their grids to bring the total idle plate current to zero as in true class B amplifiers, only one of the said screen grid tubes, when functioning, is in operation at one and the same time. Increasing the idle plate current, by reducing grid bias, causes the alternate idle tube to function to the extent of the said increased idle current, thereby increasing the overall power output. The disadvantage of this arrangement is that the tubes become overloaded as they have no rest periods as is the case of true class B audio amplifiers. A restudy of the grid and screen grid control system, as described heretofore, when applied, will show how class B screen grid push pull audio power amplifiers can be operated, in effect, at a higher idle plate current thereby increasing the overall power amplifier output, yet both plate and screen currents will drop to zero or some desired amount during said idle periods. Furthermore, should an alternating current hum appear in the output of a push pull modulator, said hum would be apparent only during idle periods. With the use of the arrangement heretofore described, no hum would appear as there would be no audio output during the said idle periods. I

In operation, a signal is applied to the grid circuit from the stage ahead causing current to flow in the said grid circuit of the screen grid tube 4 charging the electrolytic capacitor 2 to the breakdown voltage of the Zener diode 3 or voltage regulator tube, whichever may be used, thereby putting a specific and constant bias on the grid 5 of the said screen grid tube 4. After the drive is discontinued or lost, that specific bias will remain due to the ability of the electrolytic capacitor to take and hold a charge similar to that of a battery. Also assisting this function is the normal disposition of the grid of an electronic tube to accumulate a negative charge when isolated, or, in effect, isolated from its cathode. The above bias function as described, complete in itself, will maintain a constant bias with a varying input. However, normal bias alone will not reduce the plate current to zero when such is desired, so the added function of the amplifying means for the screen clamp control is necessary and its operation is described as follows.

A signal is applied to the grid circuit of the screen grid tube 4 causing current to flow through the block diagram 1, creating the bias function heretofore described, also through the resistor 18 thereby putting a negative bias on grid 17 of the triode 8, blocking its plate current so that there would be no voltage difference across its plate load resistor 16, which in turn zeroes the bias on the grid of the triode 9 thereby causing current to flow through its plate load resistor 15 which puts a blocking bias on grid 20 of the clamp tube 7 thereby. stopping the voltage reducing current drain on the screen grid of the screen grid tube 4.

When the input drive, from the stage ahead is off or lost, the bias system, as contained in the block diagram 1, continues to function as described, but the resistor 18, due to the lack of grid current flow through it, will have no voltage difference across it, therefore, in effect, grounding the grid 17 of the triode 8. Thereupon from grid 17 to grid 20 the reverse of that described above takes place causing the clamp tube 7 to become fully conductive and short the screen grid of the screen grid tube 4 to ground when the variable resistor 19 is in the shorted out position, thus bringing the said screen grid of the said screen grid tube 4 to substantially zero which in turn further assists in blocking the current in the plate circuit of the said screen grid tube 4. As the plate blocking effect of this combination is severe, the variable resistor 19 is required to manually adjust the idle plate current to just zero, otherwise it would introduce distortion when the system is used in class B modulators. The balanced resistive network 13 and 14 is necessary to equalize the plate voltage potential between the cathode and filament of the triode tubes 7, 8 and 9. If the alternating current input circuit to primary 12 of transformer 11 was grounded, which is usually the case, the total plate voltage of the secondary would appear across the cathode and filament of the triode 9. This voltage would be greater than the normal tube rating. With the application of the said resistive network 13 and 14, only one half of this said voltage would appear across the said cathodes and filaments of the said triode tubes 7, 8 and 9,

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be understood that various modifications thereof may be made within the true spirit and scope of this invention as defined in the following claims.

I claim:

1. In the input circuit of an electronic amplifier having at least one tube, said tube having at least one grid, an input grid circuit, a plate, a plate circuit and a cathode, said input grid circuit having in series a radio frequency choke, an alternating current input means and a grid bias means, said grid bias means comprising an electrolytic capacitor bridged by a voltage regulating device, all cir cuits being completed through a common ground.

2. In an electronic amplifier containing, at least, one tube, said tube having a grid and a grid circuit, a plate and a plate circuit, a screen grid and a screen grid circuit, a cathode and a screen grid draining circuit, said screen grid draining circuit having a clamping tube in series therewith, said grid circuit having in series therewith, a radio frequency choke, an alternating current input means and a resistor, said resitsor being bridged across from the grid to cathode of the first tube of a direct current voltage amplifying means, the output of the said direct current voltage amplifying means being bridged across from grid to cathode of the said clamping tube in the said screen grid draining circuit, a variable resistor in the cathode circuit of the said clamping tube to manually control the amount of conductance of the said clamping tube, all circuits being completed through a common ground.

3. In an electronic amplifier containing, at least, one tube, said tube having a grid and a grid circuit, a plate and a plate circuit, a screen grid and a screen grid circuit, a cathode and a screen grid draining circuit, said screen grid draining circuit having a clamping tube in series therewith, said grid circuit having in series therewith a radio frequency choke, an alternating current input means and a resistor, said resistor being bridged across from grid to cathode of the first tube of a direct current voltage amplifying means, the output of the said direct current voltage amplifying means being bridged across from grid to cathode of the said clamping tube in the said screen grid draining circuit, a variable resistor in the cathode circuit of the said clamping tube to manually control the amount of conductance of the said clamping tube, said grid circuit also containing a bias system comprising an electrolytic capacitor bridged by a voltage regulating device, all circuits being completed through a common ground.

4. In an electronic amplifier containing, at least, one tube, said tube having a grid and a grid circuit, a plate and a plate circuit, a screen grid and a screen grid circuit, a cathode and a screen grid draining circuit, said screen grid draining circuit having a clamping tube in series therewith, said grid circuit having in series therewith a grid bias means, a radio frequency choke, an alternating current input means and a resistor, said resistor being bridged across from grid to cathode of the first tube of a direct current voltage amplifying means, the output of the said direct current voltage amplifying means being bridged across from grid to cathode of the said clamp ing tube in the said screen grid draining circuit, a variable resistor in the cathode circuit of the said clamping tube to control the amount of conductance of the said clamping tube, a transformer having the center tap of its primary connected to the center of a balanced resistive net work, said balanced resistive network being bridged across the secondary of the said transformer, said secondary supplying the plate power for the said direct current voltage amplifying means, all circuits being completed through a common ground.

5. In an electronic amplifier containing, at least, one tube, said tube having a grid and a grid circuit, a plate and a plate circuit, a screen grid and a screen grid circuit, a cathode and a screen grid draining circuit, said screen grid draining circuit having a clamping tube in series therewith, said grid circuit having in series therewith a radio frequency choke, an alternating current input means and a resistor, said resistor being bridged across from grid to cathode of the first tube of a direct current voltage amplifying means, the output of the said direct current voltage amplifying means being bridged across from grid to cathode of the said clamping tube in the said screen grid draining circuit, a variable resistor in the cathode circuit of the said clamping tube to control the amount of conductance of the said clamping tube, a transformer having the center tap of its primary connected to the center of a balanced resistive network, said balanced resistive network being bridged across the secondary of the said transformer, said secondary supplying the plate power for the said direct current voltage amplifying means, said grid circuit also containing a bias means comprising an electrolytic capacitor bridged by a voltage regulating device, all circuits being completed through a common ground.

6. In an electronic amplifier having, at least, one tube, said tube having a grid and a grid circuit, a plate and a plate circuit, a screen grid and a screen grid circuit, a cathode and a screen grid draining circuit, said screen grid draining circuit having a clamping tube in series there with, said grid circuit having in series therewith a grid bias means, a radio frequency choke, an alternating current input means and a resistor, said resistor being bridged across from grid to cathode of the first tube of a direct current voltage amplifying means, the tubes in the said direct current voltage amplifying means being directly coupled to each other plate to grid, and each using alternate half cycles of an alterntating current source for plate power supply, the output of the said direct current voltage amplifying means being bridged across from grid to cathode of the said clamp tube in the said screen grid draining circuit, said clamping tube having a variable cathode resistor to control the amount of conductance of the said clamping tube, all circuits being completed through a common ground.

7. In an electronic amplifier having, at least, one tube, said tube having a grid and a grid circuit, a plate and a plate circuit, a screen grid and a screen grid circuit, a cathode and a screen grid draining circuit, said screen grid draining circuit having a clamping tube in series 7 therewith, said grid circuit having in series therewith a radio frequency choke, an alternating current input means and a resistor, said resistor being bridged across from grid to cathode of the first tube of a direct current voltage amplifying means, the tubes in the said direct current voltage amplifying means being directly coupled to each other plate to grid, and each using alternate half cycles of an alternating current source for plate power supply, the output of the said direct current voltage amplifying means being bridged across from grid to cathode of the said clamping tube in the said screen grid draining circuit,

said clamping tube having a variable cathode resistor to 5 control the amount of conductance of the said clamping tube, said grid circuit also containing a bias means comprising an electrolytic capacitor bridged by a voltage regulating device, all circuits being completed through a common ground.

References Cited by the Examiner UNITED STATES PATENTS 2,375,283 5/45 Cloud 330-130 X 2,767,255 10/56 Talarnini 330138 2,904,642 9/59 Quinlan 330-138 2,976,462 3 /61 Miller.

3,014,186 12/61 Webster 33024 3,040,265 6/62 Forge 330-20 X ROY LAKE, Primary Examiner.

BENNETT G. MILLER, NATHAN KAUFMAN,

Examiners. 

3. IN AN ELECTRONIC AMPLIFIER CONTAINING, AT LEAST ONE TUBE, SAID TUBE HAVING A GRID AND A GRID CIRCUIT, A PLATE AND A PLATE CIRCUIT, A SCREEN GRID AND A SCREEN GRID CIRCUIT, A CATHODE AND A SCREEN GRID DRAINING CIRCUIT, SAID SCREEN GRID DRAINING CIRCUIT HAVING A CLAMPING TUBE IN SERIES THEREWITH, SAID GRID CIRCUIT HAVING IN SERIES THEREWITH A RADIO FREQUENCY CHOKE, AN ALTERNATING CURRENT INPUT MEANS AND A RESISTOR, SAID RESISTOR BEING BRIDGED ACROSS FROM GRID TO CATHODE OF THE FIRST TUBE OF A DIRECT CURRENT VOLTAGE AMPLIFYING MEANS, THE OUTPUT OF SAID DIRECT CURRENT VOLAGE AMPLIFYING MEANS BEING BRIDGED ACROSS FROM GRID TO CATHODE OF THE SAID CLAMPING TUBE IN THE SAID SCREEN GRID DRAINING CIRCUIT, A VARIABLE RESISTOR IN THE CATHODE CIRCUIT OF THE SAID CLAMPING TUBE TO MANUALLY CONTROL THE AMOUNT OF CONDUCTANCE OF THE SAID CLAMPING TUBE, SAID GRID CIRCUIT ALSO CONTAINING A BIAS SYSTEM COMPRISING AN ELECTROLYTIC CAPACITOR BRIDGED BY A VOLTAGE REGULATING DEVICE, ALL CIRCUITS BEING COMPLETED THROUGH COMMON GROUND. 